The use of sensors to detect chemical substances both qualitatively as well as quantitatively is a growing field. Sensor use is not limited to industrial sectors. Sensors are used in air pollution studies, medical hygiene and numerous other fields.
Many of the existing sensors are electrically driven, and thus have safety problems. For example, many of the submersion sensors are electrical devices and thus have the possibility of current leakage when immersed in water. Other electrically driven sensors, such as alarm sensors that use metallic conductors, cannot be used due to the existence of an electromagnetic field.
Optical fibers have been typically used to transmit light. The theory of the optical transmission in optical fibers is complex. Generally, however, as light enters the end of a long optical fiber, some of the light will travel in a straight axial path thorough the center of the fiber, while the remaining light will come in at various reflection angles and does not follow a straight path as it is reflected from the outer surface of the fiber. The speed of light divided by the index of refraction is the speed of light in the medium. Thus, light traveling through a lower index of refraction material will go faster than that traveling thorough a high index of refraction material. Light traveling in a straight line in the optical fiber will go the longest distance. To make sure that all the light in the optical fiber travels the same distance, the index of refraction is varied throughout the thickness of the fiber. When a cross section of the fiber is taken, the index of refraction will be the greatest at the center of the fiber, and smallest at the outer edge of the fiber, so that the light traveling through the center will be the slowest, and the light traveling through the outer edge, coming in at the greatest reflection angle and thus having the most distance to travel, will be the fastest. In an ideal optical fiber, the total optical intensity of the optical input is equal to the total output intensity.
Optical fiber has been used for sensors. An example of a sensor that uses transmission loss properties of optical fibers is a submersion sensor described in a Japanese patent (Oitsukai)Showa62-25845. However, since the submersion sensors that use the optical fibers require a plate in contact with the optical fiber, the submersion sensor area is very large. This submersion sensor lacks flexibility of use. Other problems with these sensors include the high cost and the time required to fabricate the device.
Japanese patent (tokukai)Hei3-502610 describes a sensor to analyze chemical composition using optical fibers. This patent describes a commonly used theory to make sensors. The internal reflective properties of the optical fibers change as the chemical is absorbed into the fiber. The changes in the internal reflective properties results in changes in the emission intensity of the optical fiber, and that change is measured quantitatively and related to the chemical composition.
The optical sensor described in U.S. Pat. Nos. 4,492,121 and 4,542,987 contains a material that fluoresces on the end of the fiber. The fiber transmits both the excited signal and the response signal.
The U.S. Pat. No. 4,040,749 describes an optical fiber sensor with a wave guide path. A liquid crystal material is attached to the surface of the optical fiber that changes the light transmission property of the wave guide.
U.S. Pat. No. 4,399,099 describes a fiber sheath sensor that quantitatively analyzes the different reaction types of the chemicals being detected in a liquid. This device transmits electromagnetic energy, and contains one or more transmitting or semi-transmitting sheath.
The common characteristic of the optical sensors such as those described above is that they measure small changes in the reflection angles and intensity resulting from a change in the overall reflection condition in the fiber.
However, the drawback of these methods is the limit in the measurement sensitivity since these changes in the reflection angles and intensities are very small. Thus, a sensor needs to be developed which has much higher sensitivity and that can be used in wide range of applications.
For hydrocarbon and other neutral solvents, the reactivity with any other chemicals is minimal. Thus, it is difficult to fabricate a sensor which utilizes reactivity with these materials to change the optical properties of the fiber. At present, there are no simple detection methods for such substances other then very specialized methods.
In addition, there are no sensors that can constantly monitor deadly poisonous gases such as arsine and phosphine except for the use of experimental test papers. The performance of semiconductor devices is related to the concentration of gases used during the semiconductor manufacturing. However, since there is no method to measure such concentration, it is difficult to conduct research to understand the mechanism of the relationship between the concentration of gases and the device characteristics of the semiconductors. Consequently, the development of a new sensor is needed that can be applied to areas where sensors have not previously been used.
The sensor of the present invention overcomes the limitations of prior sensors by providing improved measurement sensitivity, and a wide range of optical applications including the capability to measure neutral solvents and the ability to monitor poisonous gases which was previously not possible.